Millions of obese Americans fail to achieve and maintain substantial fat losses, but a small number are successful in doing so by combining large volumes of exercise with dietary control. Although the hormonal and physiological controls of feeding under sedentary conditions have been studied in great detail, how expenditure of metabolic fuels during exercise affects these controls is poorly understood. A better understanding of whether the metabolic energy expenditure is undetectable at the conscious level, and to what extent exercise intensity may suppress appetite through hormonal mechanisms, could lead to improved strategies for fat loss in obese individuals. Based on our preliminary study and the results of others, we hypothesize that (1) changes in the concentration of circulating metabolic fuels caused by energy cost of exercise are not detected consciously as changes in the levels of hunger and desire to eat, or compensated for through changes in the quantities of food consumed, in contrast to the appropriate psychophysical and physiological adjustments when calories are withheld from meals during a day; (2) changes in the metabolic energy expenditure are metered and compensated for by autonomic and hormonal reflexes which adjust circulating metabolic fuel availability; and (3) at high exercise intensities, hormonal and/or sympathetic responses to exercise may additionally induce an active suppression of hunger. Our first specific aim is to compare how an equal energy deficit incurred through exercise or through withholding of food, either in the absence, or in the presence of intravenous nutrient replacement, affects (1) sensations of hunger, desire to eat, and estimate of capacity to eat before the meal, and sensation of fullness after the meal; (2) secretion of counter-regulatory hormones ghrelin, growth hormone (GH), epinephrine (E), and glucagon on one hand and of postprandial hormones cholecystokinin=CCK, glucagon-like peptide-1 (GLP-1), peptide YY ; insulin, and leptin, and (3) plasma concentrations of metabolic fuels glucose and free fatty acids (FFA). Our second specific aim is to compare the psycho-physical (as in 1), hormonal (as in 2), and metabolic (as in 3) responses to exercise of two intensities, at 35% and 70% of maximal effort. The findings from these studies will extend theoretical understanding of energy regulation and provide insight for improved weight-loss strategies in obese individuals.